Flexography, also referred to as flexographic printing, is a rotary letterpress printing process, traditionally using flexible elastomeric printed rolls and fast-drying inks. Flexographic printing is used for printing flexible packaging materials, including paper and plastic films, multiwall bags, corrugated containers, envelopes and paper-back books. The printing apparatus usually encompasses a reverse-angle doctor blade assembly adjacent an anilox metering roll. The anilox roll typically has approximately 100,000 cells per square inch of surface area dispersed on its outer surface. The ink distribution system operates by flooding the anilox roll with ink, thus flooding the cells on the roll's surface. As the anilox roll rotates, the reverse-angle doctor blade shaves the surplus ink flush with the surface of the cells. The result is a uniformly metered ink film applied by the anilox roll to the surface of the printing roll.
A pressure preload system for a flexographic printer usually places a doctor blade assembly in intimate contact with the outer surface of an anilox roll to control the amount of wiping action applied to the roll's surface. The doctor blade assembly is usually attached to the end wall of an ink chamber located adjacent the circumferential surface of the anilox roll.
A common problem associated with prior art preload systems is the difficulty in applying and maintaining a sufficient force to the doctor blade assembly so that the doctor blade is maintained in intimate contact with the outer surface of the anilox roll. It is crucial that the proper amount of wiping action be applied to the surface of the roll to prevent ink leakage and to achieve the desired printing quality resulting from a uniform application of ink to the printing roll, without causing excessive doctor blade or roll surface wear from unnecessarily large loading forces.
One type of known preload system employs a series of mechanical hand screws which are manually adjusted by an operator to apply a desired force to a doctor blade assembly. The screws are located on the assembly supporting the blade and apply a fixed force to the assembly. A major drawback of this system is that the screws must constantly be manually readjusted by a human operator during operation of the printing press. If the operator incorrectly sets the pressure or there is a sudden change in the conditions of the press, there is a likelihood of premature blade failure or inconsistent wiping action on the roll. There is also an obvious danger of injury to an operator who must adjust the screws while the press is operating.
A second type of known preload device applies a preload force to the doctor blade assembly by using conventional inflatable air pressure tubes or cylinders which must be manually preset by an operator to apply the desired preload force to the doctor blade assembly. The air pressure is normally supplied to the cylinders by the printing plant's in-house air pressure system. Typical air pressure systems are regulated to 50 psi or more. However, only a relatively low pressure is required to provide a preload force sufficient to maintain the doctor blade assembly in intimate contact with the roll. Low pressure air systems, however, are not practical. Inherent in low pressure air supply systems are the difficulties involved in supplying and maintaining a low pressure. Moreover, because air is easily compressible and does not readily transmit compressive forces, there is a risk that the air pressures to be applied to the blade loading system will result in widely variable preload forces on the doctor blade assembly, leading to inconsistent wiping action, excessive wear or ink leakage. Furthermore, if there is a momentary loss or reduction of the plant air pressure, there is a major risk of ink leakage or, in the case of air pressure failure, the ink chamber moving away from the roll resulting in ink flooding. Inconsistencies in the wiping action and blade failures can result in poor quality printing and lower graphic capabilities.
There is a need for a simple but effective system for applying a preload force to a doctor blade assembly that avoids these problems. The present invention fills that need.